Quote Originally Posted by Diapositivo View Post
If at 300 m we use a tele lens and we fill the image with the piano, a lesser quantity of light fills the entire image, so we have a different exposure.
Correct. The longer lens will change the size of the image, compared to, say, a shorter lens, but cannot change the amount of light it receives form the subject. So telelenses project a larger, but darker image, i.e. are slower, than shorter lenses.

(Assuming the same size light gathering area, of course. You can give a telelens a larger front lens/entrance pupil, and it will be able to gather more light than a smaller, shorter lens. It will need that - collect more light - to be able to project a larger image that is as bright as the smaller image. Make the front lens large enough, and it will be able to collect as much light as is needed to project a lager image that is also brighter than that of the shorter lens. But that shorter lens can also be made faster, etc., etc.)

Quote Originally Posted by Diapositivo View Post
If I place on the piano a grey card and read it with a spot reflected meter, which only reads light reflected from the card (that is, supposing the card "fills" the reading angle of the spot lightmeter), my understand of physics tells me that my spot meter will give me a different exposure than the incident light meter used near the piano.
Assuming that there are no other variables, it does not.
Why would it?

Quote Originally Posted by Diapositivo View Post
If I take a picture of a lit monument at night, the exposure for the monument isn't the same even if I am far from the monument.

I'm really puzzled.
The answer is in what is said before, and what was quoted from that encyclopedia.